U.S. Pat. No. 3,081,519 to Blades and White describes a flash-spinning process for producing plexifilamentary film-fibril strands from fiber-forming polymers. A solution of the polymer in a liquid, which is a non-solvent for the polymer at or below its normal boiling point, is extruded at a temperature above the normal boiling point of the liquid and at autogenous or higher pressure into a medium of lower temperature and substantially lower pressure. This flash-spinning causes the liquid to vaporize and thereby cool the exudate which forms a plexifilamentary film-fibril strand of the polymer. Preferred polymers typically include crystalline polyhydrocarbons such as polyethylene and polypropylene.
According to Blades and White, a suitable liquid for flash spinning (a) has a boiling point that is at least 25.degree. C. below the melting point of the polymer; (b) is substantially unreactive with the polymer at the extrusion temperature; (c) should be a solvent for the polymer under the pressure and temperature set forth in the patent (i.e., these extrusion temperatures and pressures are respectively in the ranges of 165 to 225.degree. C. and about 500 to 1500 psia (3447-10342 kPa); (d) should dissolve less than 1% of the polymer at or below its normal boiling point; and (e) should form a solution that will undergo rapid phase separation upon extrusion to form a polymer phase that contains insufficient solvent to plasticize the polymer.
Commercial spunbonded or flash-spun products have been made primarily from polyethylene plexifilamentary film-fibril strands and have typically been produced using trichlorofluoromethane as a spin agent; however, trichlorofluoromethane is an atmospheric ozone depletion chemical, and therefore, alternatives have been under investigation. There have been many other agents used for flash spinning polyethylene to either minimize or eliminate the potential for ozone depletion. Shin, in U.S. Pat. No. 5,032,326 discloses one alternative spin fluid, namely, methylene chloride and a co-spin agent halocarbon having a boiling point between -50.degree. C. and 0.degree. C. Kato et al. in U.S. Pat. No. 5,286,422 discloses an alternative, specifically, a spin fluid of bromochloromethane or 1,2-dichloroethylene and a co-spin agent of, e.g., carbon dioxide, dodecafluoropentane, etc.
As noted above, flashspun products have typically been made from polyethylene, however it is desirable to make flashspun products from other polymers, such as polymethylpentene that have the advantage of a higher melting point than polyethylene.
U.S. Pat. No. 5,250,237 to Shin mentions the use of alcohols with one to four carbons as spin agents for flash spinning polymethylpentene. Also, in a co-pending application assigned to DuPont 09/211,822 filed Dec. 15, 1998, certain azeotropic mixtures are used as spin agents for polymethylpentene. Regardless, a need exists to find additional solvents suited for polymethylpentene, yet also satisfy the need for non-flammability and zero or extremely low ozone depletion potential.